1. Field of the Invention
The present invention relates to an apparatus for providing selectable, multi-colored back-lighting for liquid crystal displays (“LCD's”).
2. Description of Related Art
Liquid crystal materials emit no light of their own. They do however reflect and transmit light from external light sources.
Accordingly, it is necessary to back light the display, which is typically done with fluorescent lamps located either directly above or behind the LCD or on either side thereof. When illuminated from behind, a white diffusion sheet between the backlight and the LCD redirects and scatters the light evenly to ensure a uniform display. When light is transmitted through filters, layers of liquid crystal, electrode layers, polymer films, etc., more than eighty percent (80%) of the light is lost.
Moreover, in certain applications it is preferred to have back lighting for LCD's of only certain colors, such as in the cockpit of an aircraft, where red light is undesirable for use with night vision equipment. For clarification of the discussion below, the following definitions for Type I and Type II, Class A and Class B Night Vision Instrument Systems (“NVIS”) lighting conditions are taken from the military specification document MIL-L-85762A, Section 1.3.
1.3 Classification. NVIS compatible aircraft interior lighting shall be of the following types and classes, as specified:    Type I Lighting compatible with and Direct View Image NVIS utilizing generation III Image intensifier tubes.    Type II Lighting compatible with any Projected Image NVIS utilizing generation III Image intensifier tubes.    Class A Lighting compatible with NVIS utilizing 625 nm minus blue objective lens filters.    Class B Lighting compatible with NVIS utilizing 665 nm minus blue object lens filters.
Assume for example that a pilot is wearing Type 1, Class A NVIS goggles, which are used for detecting infra red images in a limited-light or dark scenario. Now suppose internally reflected light or instrument panels of the aircraft comes within the field of view of the night vision goggles. If the displays are back lit with Class B filtered light, as is typically done for multicolor displays, then red light emissions at 665 nm wavelength emitted by the Class B displays will reduce the gain of the night vision goggles for a pilot wearing Type 1, Class A night vision goggles. The pilot's ability to use his Class A goggles to detect external IR images will be seriously impaired.
Type 1, Class A goggles are used for green monochrome displays and lighting. For such displays, no amount of red light emissions are acceptable, such as color displays having red as a part thereof. Hence, the liquid crystal display cannot be used in the multi-color mode. To overcome this problem, optical filters have been used but with a significantly lower light transmission compared to one that is used for a night vision multicolor display.    Type I, Class B night vision goggles are designed for use with multicolor displays where a limited amount of red spectrum is permitted, as per military specification MIL-L-85762A.
Class A goggles are designed to view external images at night at peak wavelength as low as 625 nm wavelength. In order for the display and cockpit lighting to be compatible with Class A goggles, the IR and red components must be completely removed with filtering, thereby rendering the display unsuitable for displaying red colored data at night.
To overcome the above-described problem, Class B goggles were designed for limited external use to wavelengths as low as 655 nm only. Hence, red data could safely be displayed because the Class B NVIS filters are designed to pass a limited amount of the red spectrum.
Today, LCD backlighting systems do not allow the display to be used for both Class A monochrome and Class B multicolor NVIS applications utilizing the same hardware. An NVIS optical filter can be designed for either Class A or Class B operation, but not both.
Therefore, a need exists for a display that can be switched from one color back lighting to another in order to adapt to a variety of situations or night vision systems.